The current market for products made from soft absorbent paper webs has long been split between premium products and economy products. Commercial paper toweling, dispenser napkins and single-ply tissue products are often relegated to the economy value market because they have often been made from inexpensive recycled fibers resulting in thin and/or rough products, often having poor absorbency. It was heretofore difficult to make soft absorbent paper webs having sufficient strength, softness and absorbency to qualify as premium or near-premium quality without resorting to more expensive virgin fibers and/or expensive processing methods.
Through air drying (TAD) has changed the industry's ability to produce soft, bulky, premium quality paper products, particularly in the area of single-ply products. TAD has become the preferred choice for newly purchased paper machines because it can provide improved product attributes and therefore, economic advantages to manufacturers when compared with the products produced by conventional wet pressing (CWP). The advent of TAD has made it possible to produce paper products with good initial softness and bulk.
In the older conventional wet pressing method, premium quality paper products, tissues and towels, are normally made by embossing together two thin plies. In this way, the rougher air-side surfaces (i.e., those surfaces not previously in contact with the surface of the Yankee dryer) can be made to face inward, thereby being concealed within the two-ply sheet. However, embossing two-plies together imposes marked economic disadvantages over single-ply paper TAD sheets.
Conventional wet pressing, however, has certain advantages over TAD including 1) lower energy costs associated with the mechanical removal of water rather than drying by the passage of hot air; and 2) increased production speeds. Stated differently, energy consumption is lower and the production speeds can be considerably higher than those used in TAD.
Conversion of existing CWP machines to TAD capability is both difficult and expensive. What is needed is a method of making premium quality, or near-premium quality paper products using conventional wet pressing from recycled fiber. More preferably, a premium quality or near-premium quality two-ply and even more preferably a single-ply product should be produced from inexpensive and recycled fibers without the need for significant preprocessing of the fibers to remove ash and fines.
Attempts have been made to produce products from recycled fiber using CWP that can compete with TAD products, but these processes often suffer from limitations making it necessary to use more expensive virgin fibers to achieve an acceptable product. One common method of increasing the softness and cushion of bathroom tissue is to crepe the paper. Creping is generally accomplished by fixing the cellulosic web to a Yankee drier with an adhesive/release agent combination and then scraping the web off the Yankee by means of a creping blade. Creping, by breaking a significant number of inter-fiber bonds, adds to and increases the softness of resulting bathroom tissue product. However, creping with a conventional blade may not provide the most preferred combinations of softness, bulk and appearance.
According to one preferred embodiment of the present invention, we have discovered that tissue having highly desirable bulk, appearance and softness characteristics, can be produced by a process similar to conventional processes, particularly conventional wet pressing, except that the conventional creping blade is replaced with the patented undulatory creping blade disclosed in U.S. Pat. No. 5,690,788,1 presenting differentiated creping and rake angles to the sheet and having a multiplicity of spaced serrulated creping sections of either uniform depths or non-uniform arrays of depths. The depths of the undulations are above about 0.008 inches. 1U.S. Pat. No. 5,690,788 is herein incorporated by reference in its entirety.
The present invention makes it possible to use inexpensive secondary fiber that may contain significant amounts of ash and fines and yet, achieve a premium or near-premium quality paper product. The paper products made according to the present invention exhibit characteristics approaching the much more expensive TAD products. Moreover, products made using the patented undulatory blade to crepe the web will have a crepe fineness similar to that of conventionally-made tissue sheets, but the resulting web combines crepe bars extending in the cross direction with undulations extending in the machine direction. The resultant product will have a lower tensile strength and a higher caliper and cross-directional stretch than is found when using a conventional crepe blade.